(Also, on a quieter note, if you're using a homebrew style media centre then where is the media coming from? You're either ripping your own - in which case you might as well just stick in in a DVD player - or, uhm, getting it from somewhere that probably isn't packaging it in a 5.1 format...)
Part of the reason for using a media centre is convenience... I dare say many people will these days be ripping DVDs/Blu-Rays to network storage.
Also, can anyone chime in on audio support on the SoC side? I'm assuming 5.1, no fancy stuff (DTS, bitstreaming, etc.), something on par with the 1st-gen Ion chipset, which is not that bad, but please remind me, if possible.
The way I look at the R-Pi Model B is that it is a Nokia N8 on steroids. The R-Pi/B has a slightly more powerful CPU and a much more powerful GPU than that present in the N8.
The Nokia N8 can "only" manage 720p and Dolby Digital Plus (in theory 7.1 but in practice likely to be 5.1) audio, whereas the more powerful R-Pi GPU is capable of 1080p30 so I don't see why the R-Pi can't also handle at least 5.1 digital audio.